This invention relates generally to electric motors, and more specifically, to start capacitor assemblies and methods for operating electric motors.
Some known types of electric motors, sometimes also referred to as dynamoelectric machines, include a start winding and a run winding. The start winding is utilized to initiate rotation of the motor rotor. When the run and start windings are energized, the phase relationship between magnetic fields generated by the run and start windings, and the magnetization of the rotor, cause the rotor to begin rotating from a standstill condition. Once the rotor has sufficient torque to attain its normal running speed, the start winding is typically “switched out” of the motor circuit. In some other motors, the start winding is not switched out of the motor circuit.
Start capacitors sometimes are utilized to change the time phase relationship between the magnetic fields generated by the start windings. A start capacitor connected in series circuit with the start winding causes the magnetic field generated by the start winding to be out of phase with the run winding field by a greater degree. The greater phase shift of the start winding magnetic field results in a higher starting torque. A disconnection device, such as a centrifugal switch or a current sensing device, typically disconnects the start capacitor once the motor reaches full operating speed.
Start capacitors are commonly electrolytic capacitors. Electrolytic capacitors exhibit relatively high electrical losses that results in relatively high operating temperatures that may decrease reliability of the capacitor. To avoid overheating and damaging electrolytic start capacitors, some electric motors are designed to ensure that the electrolytic start capacitor is disconnected after no more than 2.5 seconds. The high electrical losses and heat generation of electrolytic start capacitors may also be of concern in applications in which a motor is cycled on and off fairly quickly because there may be insufficient time for the electrolytic start capacitor to dissipate the heat generated during startup of the motor.